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Why is a control surface that allows two sides of airflow better than those that allow one?
For example, ailerons allow air to flow past both the top and bottom surfaces, which makes it more aerodynamic than speedbrakes. Also, plane rudders have air flowing on both sides, which is apparently mote efficient than a spoiler. Why does this happen?

For low drag, you want to minimize the surface changes presented to the air flow. Large changes in the surface will create turbulence and pressure changes that contribute to drag. Small changes will minimize these sources of drag.
When you move a control surface like an aileron or elevator, air is flowing past both sides of the airfoil. Although this changes the direction of the flow, which gives the control force as well as some drag, the air can still flow past both sides.
In the case of a spoiler, the air cannot flow past both sides. This increases the frontal area presented to the flow, and creates a low pressure area behind the deployed surface. Creating a low-pressure area behind an object is a big contributor to drag, which is why aerodynamic bodies tend to have a teardrop shape.
Spoilers on the wing also have the effect of reducing lift. By disrupting the smooth low-pressure flow over the upper surface of the wing, they reduce the effectiveness of the wing, effectively stalling the affected portion.
For an illustration of both principles in action, see this video of a speed brake on a Mooney. This shows how even a relatively small object, when disrupting the air flow, can add a significant amount of drag.

"This increases the frontal area presented to the flow, and creates a low pressure area behind the deployed surface." If you look at a extended aileron from the front, doesn't it also take a lot of frontal area?

@EwenW. Yes, but the flow is allowed to turn on both sides, so from the direction of the flow, the frontal area is the same. No new external area has been created.

The reason is that both sides of a control surface experience pressure  changes, both ahead and aft of the hinge line. In case of spoilers and speed brakes, only one side is affected, so the effectiveness is only half as big.
Please see the pressure coefficient plots of an airfoil for three different flap deflections below. Upper and lower surface pressure are shown by color-coded lines, and lower lines belong to the lower surface. Dashed lines show the inviscid pressure, and solid lines the pressure distribution with friction effects added. The wider two lines of the same color are apart, the more lift is created. Note the contour plot below, which follows the color scheme of the pressure plots.

While control surfaces normally are deflected by less than ±20°, regular spoilers are extended by 60° or more to create as much drag as possible. But even if they are extended by similar angles to those of a control surface, the air will have no chance to flow orderly along the leeward side. Instead, this side will only have separated, chaotic airflow, while the windward side will show a flow pattern very similar to the windward side of a control surface. The pressure in separated flow will not change with deflection angle, so this side will not contribute any lift effect.
Note in the picture above that the pressure level over the whole chord is pushed up or down by the flap, because the flap is located in the aft portion of the airfoil. In contrast to that, spoilers are mounted mid-chord, where their influence over the pressure level is smaller. This reduces the lift effects of spoilers even more.

The main answer is drag. A spoiler will create more drag than a surface that allows flow over both sides. In the case of speed brakes this is what you want to happen as you want to slow the plane down. With control surfaces you want to alter airflow with out creating excess drag.

I think you question comes to the difference between aerodynamic body and blunt body. It is no so much linked to exposing one or two surfaces to the external flow, but more linked to the shape exposed to the external flow.
An aerodynamic body has a shape which produces low drag, aerodynamic bodies have a very small size normal to the direction of flight. For example, the wing is really thin in the vertical direction, but huge is the direction the plane is expected to flight.
A blunt body is the oppossite way around, exposes a big surface in front of the moving direction. The spoilers are a very good example.
I can give you a practical example, you hand. When you are in your car (in movement) and you extend your hand outside the car, having the hand parallel to the ground, you don't notice significant "pressure" or "force" over your hand (please use some nice speed...) however is you set your palm looking to the direction the car is moving, the "pressure" over you hand is much bigger as you are exposing a bigger surface.
Example the same phenomena with your hand is happening with ailerons, rudders and airbrake.
We use an airbrake to stop the airplane, that is why we use like your palm in the car. Stop the airplane.
Ailerons and rudders are intended to not stop the airplane, but control it. That's why the intention is to have low drag.


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Crash-landing in the sea
A recurring theme I come across is how, in an emergency, it is safer for a passenger craft (such as an Airbus) to try to land on the ground rather than at sea. My understanding is that the main reason for this is that at the high speed at which the plane would be travelling, landing in the sea would be like hitting concrete and basically break the plane up and kill most of those on board.
But if speed is the issue then why cannot a passenger craft just have a large parachute at the back that is deployed when the plane is say thirty to fifty feet or so above the sea surface so that the speed is drastically reduced (at a carefully pre-calculated rate that is safe for all on board, who would in any case be fully prepared for the landing), at which point the plane would essentially just guide gently into the sea at a speed maybe equivalent to that of a fast speedboat?  To be clear I am talking about a parachute that is just intended to add drag and slow the plane down.
Would this work in practice, and if so why don't planes have such a parachute as standard?

If the parachute reduces the speed quickly, everyone's injured by being thrown forwards; if it doesn't, the plane drops below its stall speed and falls out of the sky.

The comparison of water to concrete limps on both legs. Water is obviously softer than concrete and is so at any speed. However when making controlled emergency landing, the hardness is not what matters, because the vertical speed is low. What matters is roughness. The surface needs to be flat so the plane can slide on it. So rivers and lakes are usually decent option for emergency landing, but sea always has waves and the plane will slide into the valley between them and hit the next wave hard which is likely to break it.

On the other hand, in the short timespans of collisions, water is harder than concrete because it cannot compress as well and displacement in large object high speed collisions is negligible.

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Where can I download flight data containing pitch, roll and yaw angles?
I'm researching the amount of solar energy available to photovoltaic cells placed at various locations on an aircraft's body. To calculate the angle of incidence of the sunlight on different parts of the plane I need a complete set of attitude angles of the aircraft (pitch, roll and yaw or equivalents) during flight. Can anyone tell me where to get this data?

get on a flight with a gyro and log the data?

Hi Rebecca, welcome to Aviation.SE. I am voting to close the question as too broad, as pitch, roll and yaw will depend on too many factors, e.g. aircraft type, departure and destination airport (route flown), takeoff weight, distance of flight (and thus, fuel load). The values will be different for each flight.

Hi, thanks for the quick response. I don't think the question is too broad as I'm only looking for leads on sources of flight data that already contain this information. I'm not trying to generate the data from first principles. I just need realistic flight data to test out my model.

Hi Rebecca, you are assuming that such data is available, I do not think it is, as it would not make sense (or it could be sensitive information if recorded during test flights).

Welcome. Provided you're aware that sun exposure will depend on the overall direction of the flight (e.g. N-S vs E-W), I think you can obtain data for a given flight by using a computer flight simulator. You may want to get touch with such users community, e.g. AVSIM and X-Pilots. They may be able to record a flight and provide the attitude data set.

Good lead - I'll check out AVSIM and X-Pilots.

I suspect what is important is that planes spend 99% of their time straight-n-level.  Even when they bank, they typically bank less than 30 degrees, and pitch less than 20 degrees, and that is only for relatively short times.  (I don't see how yaw affects sunlight incidence, as it is effectively just flying on a different heading)

It's true, level flight accounts for most of the time but you could be in a holding pattern for a few hours.

The yaw doesn't really affect incidence if the PV cells are flat but if they are tilted it could make a difference. For example if the sun was coming up and the cells were facing west due to the plane's heading.

Voted to reopen, because this has generated a lot of good discussion.  Strongly agree with the idea that most of your time is spent straight & level (about 4-5 degrees positive pitch compared to level-on-the-ground attitude), and over the course of lots of flights, two things will far overshadow the effects of the time that you spend in turns: cloud cover, and direction of flight (N-S vs E-W).  For every flight that spends an hour holding (10-20 miles straight at 200 kts, turn 180 degrees at 20 deg bank, repeat), another will spend time under cloud cover.

To continue the reasons to reopen, there ARE some good answers to the question as stated, although for the OP's purposes a simulation or log like FlightRadar24 (MikeFoxtrot's answer) is FAR easier & cheaper, and almost certainly plenty accurate. Foreflight (coupled with an AHRS) can record exact pitch/roll/position/track values for a flight, but only if you set it up on that flight ahead of time. Airlines capture data for EXACTLY these parameters + far more as part of FOQA data acquisition, but getting access to that as an outsider is probably impossible. Worthwhile discussion to have, tho.

Searching for simulator add-ons specialized in recording flights, I found TacView. On their site there are samples of recording in CSV format. You may have a look under section Real Life, FSX, X-Plane, One CSV file can be downloaded as Real life .csv files Demo. It contains the information you are looking for.

To answer the question as asked, such data is out there, although it may be hard to get.  A flight simulator program is probably much easier, although the real answer to the question probably is, the effects of anything other than straight & level flight are probably dwarfed by the effects of DIRECTION of flight coupled with time-of-day (i.e. is the sun overhead, or low on the horizon to one side or the other, or ahead of or behind the aircraft), and the effect of cloud cover.
Airlines capture exactly the data being discussed as part of the Flight Data Recorder -- one of the two famous "black box" recorders (the other being the Cockpit Voice Recorder).  This data is routinely downloaded by the airlines' maintenance departments and analyzed as part of Flight Operations Quality Assurance or FOQA programs.  They can see roll, pitch, yaw, heading, altitude, position, track, and dozens if not hundreds of other parameters, all on a moment-by-moment basis, and the analysis of this data is used to inform any number of proactive safety efforts.  Rather than waiting for an accident, they can see where things are starting to drift away from what's desired, and take preventative actions.
The problem here is that FOQA data is extremely sensitive, and for an outsider to get access to it is probably somewhere between extremely difficult, and absolutely impossible.
The next step down from that would be the logged data as has been discussed in the various comments, and I'm not smart enough on those specifics to add anything useful to what's already been posted as far as where and how to get those logs.
After that, it is possible to record data yourself, with the right equipment (not cheap).  Using an iPad with Foreflight and an AHRS unit (essentially a self-contained INS, with receivers for things like XM satellite weather, GPS, and ADSB, you can record a track log that includes all the parameters mentioned.
Here's the issue: if you're paying for that equipment and the flight time, you will want to get DATA for your efforts... so let's go do STUFF so we have an interesting track log: do some holding, fly a few approaches, maybe a steep turn or a stall series.  Because that's much more interesting DATA!
But the simple fact is that when airplanes are actually being used for anything besides flight training, most of what they're doing is simple boring straight & level flight, probably on autopilot.  Most of the maneuvering happens at the beginning & end of the flight, on the departure and then on the arrival and the approach, and that all happens at lower altitudes where clouds will have the greatest influence how much sun your solar collectors are getting.  Up in the 30-to-40-thousand feet range is where most of the cruise flight happens, mostly straight and level, and with most (though not all) of the sun-blocking clouds below you.
For a realistic estimate of how much solar energy your panels can get, I strongly suspect that you'll do best with an assumption of straight & level flight, and models of the various possibilities of direction of flight and time-of-day.  And combine that with estimates of how cloud cover will affect your ability to collect solar energy (remembering that plenty of flights happen at night).
A really accurate log of a particular flight could allow you to calculate to fairly high precision how your received energy would vary over time for that exact flight, but when you want to generalize that out to a large number of operations, you'll need to look at the factors that influence collection the most, and those will be the variables other than detailed records of roll & pitch.

FlightRadar 24 has an API that you can get data from. This is only from aircraft with ADS-B transponders, so it excludes some smaller and older planes. This would indirectly:

Give you points with which you could calculate the turns.
Give you pitch as the rate of climb.

This might give you a bigger but less exact dataset. Unfortunately, you might be disappointed in that normal commercial aircraft make relatively few manoeuvres at cruise.
Depending on the sort of simulations you might find that making a simulated flight in FlightGear (Open Source Simulator) might yield better results. If memory serves me right there is some equivalent to a flight data recorder so you can pull the data when you are finished.

I'm giving FlightGear a try. Thanks to MikeFoxtrot and everyone who gave me pointers above. I think I have enough leads to find the data I need.

ADS-B data doesn't seem to include attitude, according to this SKYbrary page. Do you confirm it is part of the technical specifications?

@mins Eurocontrol says it should be.

@MikeFoxtrot: Thank you for the link. You are right, and ADS-B seems pretty rich regarding information broadcast.

Assuming that you are asking this question to come to an optimum design point and conclude on the likelihood of different attitudes that can be seen by an aircraft.

Yaw angle (heading) is the simplest part: as aircraft fly all around the world, you can take all possible heading angles into account. (Reasoning: aircraft fly to all directions possible. If you need to go into statistics of the subject, you can multiply the number of flights for each of the aviation hiways.)
For Pitch angle, you can assume that during climbout, depending on the aircraft specifications, there will be an angle from a few degrees to ten plus degrees (larger climb rates involve military aircraft). And during descent, a negative angle of similar amount would take place. During cruise, pitch angle is close to zero: might change zero point some degrees as fuel is burnt and when the aircraft gets lighter. Therefore, a pitch angle of zero should be the optimum design point for any solar-harnessing system on an aircraft. Please keep in mind that the aircraft body (fuselage, wings and tails) surfaces have local surface normals.
For Roll angle, unless you are looking for a loitering platform (where the aircraft turns around a location for long periods of time), the roll angle will be very close to zero for the straight legs of the flight. Turns will occasionally occur with bank angles of 20 to 30 degrees (again, depending on the radius of the turn, and the flight characteristics of the aircraft). Turns take up such a small fraction of the flight time that they can easily be ignored.

I'd like to also refer back to your main question, of where to find flight data. In case you really need it, there are online flight simulators, and their servers may have pre-recorded data. Otherwise, plugging into a server and recording the live data of flying aircraft could be considered.
Time and location of the aircraft is one other important factor for determining the sun's angle relative to the aircraft.

For time, you can assume any time of the day and year.
And for location, only the poles are not covered (there are meteorological stations up to 72degrees North,and 52 degrees south). Existing flight routes (please see "enroute charts") can be utilized to have a better understanding of where aircraft flights take place.

Note that, military and commercial flight data (attitude) is generally considered confidential and proprietary.

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What would happen if one of the engines failed during takeoff and other engine is on full power?
Assume the left engine failed and the right engine is at full power. The pilot has pulled his nose up and turned towards left. Is it possible for him to land the plane on a runway to his left? The plane is an ATR-72 turboprop design

Why would you even try?  Just continue the take off and come back for a nice safe arrival.

I want to know the SOP in such cases and if the above method is allowed

If you're already in the air, you wouldn't try to land even on the same runway, let alone on a parallel one. You've already used up most of hte tarmac. Once you're flying, you've already won the first round of the game and can almost certainly circle the airport the airport and come back in to land, or fly straight-ish to another airport, if there's one close enough.

No, no, by all means don't turn left. See also Why should you not turn in the direction of an inoperative engine?.
Transport category aircraft are capable of climbing out on one engine. It is a certification requirement for them. So if the left engine failed after rotation, the first thing the pilot has to concentrate on is to keep the plane flying and under control. That means ease the pull on the stick a bit to avoid stalling as the plane is not able to climb as fast and push the right rudder pedal to keep the plane flying straight¹.
Once the plane is in straight climb at $V_2$ speed, it's time to deal with the engine. First both pilots cross-check the instruments and the rudder position to determine it was the left engine that failed. Then they feather the propeller (auto-feather might have done it already) and cut fuel to the failed engine. If the engine fire warning is on and did not extinguish when fuel was cut, they discharge fire extinguisher.
These things are done according to standard emergency procedure that the pilots have to remember and train in simulator. I don't know the specifics for ATR-72, but in all aircraft it is roughly like above.
Meanwhile the aircraft climbs to safe altitude and then they should level off and let it accelerate a bit to get some safety margin for manoeuvring.
The plane is now in relatively safe condition. Even if engine was on fire, it is unlikely to spread while the aircraft is flying. So the pilots now have time to declare emergency and ask the controller for what options they have for landing. Whether they do a complete circuit and land back on the same runway or turn around and land in opposite direction or land on some other runway then depends on wind and where they are relative to the runways at the time.
There were many accidents where plane crashed after one engine failure during take-off because pilots started manoeuvring before they attained sufficient altitude and safe speed. It is better not to hurry, engine failure does not require it.

¹ Of course provided there are no obstacles in the way. For example if you were taking off from VQPR runway 33 and your right engine quit, you'd still have to do a right turn rather early to avoid the Mr. Smith's house right in the axis of the runway and quite a bit above it. But for any airport that require such avoidance (and there is quite a few of them in various mountains), the pilots are specifically briefed and trained for that airport which includes instructions on how to best handle engine failure during take-off and during approach and viability of the procedure is verified in simulator and with a test flight. The above answer just outlines the generic procedure.

Too broad a reply. There can be a few good reasons to turn into the inop engine. Not many, but "never" is too broad. This is one of the many typical cases of regurgitating a mantra. Sorry, I'd never fly with you.

@AndreasLauschke that seems a bit strong, especially since the word "never" isn't in this answer. Can you provide an example of when or why you should turn into the dead engine?

@AndreasLauschke This post doesn't say to never turn into the inop engine, it says not to do so in this particular case, which is completely correct. The linked question does discuss why it's generally a bad idea, though. How bad of an idea it is will depend on the type you're flying.

@egid, well, if your engine quits on take-off from VQPR, you will have to turn into the dead engine sooner or later, because there is only one way to fly the circuit and it involves turns in both directions. But pilots who fly in that airport get specific briefing and training that includes instructions on how to best fly the circuit on one engine. And it still involves getting some altitude and speed first.

The aircraft is able to fly on one engine. Sudden manoeuvring immediately after an engine failure on take-off is not advisable; the margin from the minimum control speed with one engine out (Vmc) is low and assumes a 5 degree bank into the working engine. The abnormal operating procedures of the ATR-72 advise the crew that:

The conduct of any emergency procedures or the analysis of any
  technical problems should not normally take place until:
a.  The vertical and lateral flight path is under positive control;
b.
  Possible ground contact is no longer a threat; and
c. Altitude is
  above 400 FT (AGL)

When the above conditions are met the engine shutdown procedure should be followed. These actions are to be memorized by the crew instead of being read from a check list as they are critical.
After the engine is shutdown the crew can assess the situation and decide on further actions. Returning to the departure airport will be the normal course of action but if circumstance dictate so the pilot in command may decide to land on an alternative airport.

Following an engine shutdown a two engine aircraft should normally proceed to the nearest suitable airport, although there is no objection to the aircraft proceeding to the destination provided the increased flying time is not significant.

The quotes are from an ATR-72 SOP book I found on scribd

The PIC has the authority to do whatever he/she deems necessary to control the situation. Landing on another runway on the same airport is one of several choices. Not all airports have multiple runways, and if they do, not all are suitable for an immediate landing, but if the runway layout allows for it, then yes, landing on another runway is a viable option. But only one of many. I always impart in my ME students to consider landing on another runway in case of an engine failure upon take-off, IF (and only if) the runway layout is suitable for that. On most airports, landing on another runway is not an option.

There is an emergency procedure for engine failure on take-off after $V_1$ and transport pilots train it in simulator twice a year.

I'm not suggesting, but saying that it is an option for the PIC. You cannot abrogate the PIC's choices with anything.

Fine, but that means the answer to the question "is it possible ..." is "yes". And 91.3 remains untouched, and no emergency procedure / SOP can supersede it. Any emergency procedure / SOP is subsidiary to the PIC's decision to supersede it qua 91.3

I'm afraid I have to disagree. 91.3.b provides waiver for legal rules, but not for laws of physics. Manoeuvring just after take-off into a dead engine (and note the left engine is the critical engine on the ATR-72) is a recipe for disaster.

Again, my argument is based on the assumption that there is a convenient runway at the left, as is the premise of the original question. Obviously, if there is no convenient runway at the left, you can't do that.

I've attempted to clean up this toxic comment thread without losing all of the discussion it contained. @AndreasLauschke: take it to chat next time, please, and try to be less combative.


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How are rocket engine nozzles able to survive very high temperatures without melting?
The temperature of the gaseous products exiting from the nozzle is higher than the melting point of the nozzle material. So how do the nozzles survive without melting despite exposure to such temperatures? What is the feature which enables them to do so?

Do you have any reference for the specific conditions you are interested in?

For future reference, rocket stuff has a separate site on SE - https://Space.stackexchange.com

@DeerHunter: Rockets are not all spacecraft, therefore not all on topic at Space.SE.  (Similarly, rocket ground vehicles are off-topic both here and there)

There is actually quite a lot of information on the subject in the Braeunig web site Basics of Space Flight. To give the salient points:
Liquid-fueled rocket engines
Both the nozzle and the combustion chamber itself need to be cooled. Although most of the thermal energy produced is ejected with the exhaust, some of it will indeed push hardware temperatures up if not checked. Techniques include:

Regenerative cooling, where the propergols (both propergols, or just the fuel) are pumped through a jacket around the nozzle before going into the combustion chamber. This cools down the nozzle, and heats up the propergols that may be cryogenic in nature. This is what was used in the Saturn Vs' Rocketdyne F-1 engines, and the Space Shuttle main engines.
Dump cooling, similar to the above but the fuel used to cool the nozzle is dumped overboard instead of being fed into the combustion chamber and being used as, well, fuel.
Film and transpirative cooling, where a thin film of coolant or fuel is created near the chamber wall.
Ablative cooling, where part of the combustion chamber or nozzle wall is sacrificed. As the material is melted or otherwise burned away, it absorbs some of the thermal energy, thus saving whatever is behind it.
Radiation cooling, where nozzle design is such that heat has time to be evacuated through the part's exterior wall. This is used mainly in small engines, not main.

Solid-fueled rocket engines
No information available. Nothing. Zilch. Nada.
It would seem that the nozzle and combustion chamber materials just need to be strong enough to withstand the heat from the burning gasses. It is perhaps worthwhile to note that solid rocket engines are designed to burn just one single time, and for a relatively short period of time (even the Space Shuttle boosters were extensively overhauled before being put back into service, with any necessary parts changed). Rocket engines need to withstand working conditions for just minutes, instead of the thousands of hours an A/C jet engine turbine would be expected to work without failure.
It would also be reasonable to assume -though this is strictly something I am making up on the go, with much hand waving- that of the techniques described above for liquid-fueled rocket engines, at least two could be easily adapted to solid engines:

Film cooling, with an extra tank of cooling liquid on board.
Ablative cooling, with some refractive material used to coat the inner surface of the nozzle.

@RossMillikan Good to know. A reference I could build into the answer?

Solid rockets
The environment of a solid rocket nozzle is indeed very harsh.  In 1975 Nasa published SP-8115 specifically about solid rocket motor nozzles:

...design values a designer might encounter: throat diameters from about 1/2 inch to nearly 90 inches, motor pressures from under 400 pounds per square inch to 2000 pounds per square inch, expansion ratios from less than 4 to over 50, firing durations from less than one second to 200 seconds, thrust from a few hundred pounds to over 5 million pounds, flame temperatures from 5100° F to over 6000° F, and a wide variety of propellant compositions.

While advances have been made in material and propellant sciences, this serves to illustrate the difficulty in designing rocket nozzles to withstand these pressures and temperatures without melting.
At the time nozzles were made out of a single piece of molded polycrystalline graphite, supported by metal housing structures.  They eroded (ablative) but were low cost.
Tungsten was used for a time, but it was heavy and often cracked.
Later advances led to high strength carbon fiber and carbon matrix, often referred to as a "carbon-carbon" material.
Carbon-Carbon
Also called reinforced carbon-carbon, it is used today in a wide variety of industries, from the leading wing edges and nose cones of spacecraft and ICBMs, to formula one race car brake rotors. In some forms it is well over $100,000 per square foot.
Not only does it withstand high temperatures well, it can withstand thermal shock and has a low coefficient of thermal expansion.  This means a significantly reduced risk of cracking due to thermal stress.
It is a brittle ceramic, though, with reduced impact resistance compared to some high temperature materials.  It was this material that broke on the Space Shuttle Columbia due to an insulation strike which ultimately led to the later shuttle destruction on re-entry.
In a rocket nozzle, impact resistance is not an issue - thermal shock and temperature is, and so this material is ideal for this use.  Even so, it is still ablative, and is not considered re-usable.  Re-usable rockets, however, are almost always of the liquid variety for re-fueling purposes, and those nozzles are liquid cooled from the cryogenic fuel sources.
Sources:

Solid Rocket Motor Nozzles.  NASA SP-8115
Rocket Propulsion Elements
By George P. Sutton, Oscar Biblarz

In some cases the fuel itself will be cycled through tubes in the nozzle to keep the nozzle within a defined operating temperature range.
That's the case with the Rocketdyne F-1, for example.
Edit
I just noticed a comment referring to this practice. The F-1 is the actually the engine that was used on the Saturn V.

Edit - that is correct. And the SSME seem to be a further development of the same, also built by Rocketdyne.

I remember watching this video from Youtube that provide most of the science and technologies needed to build a space shutter. And it also explained how to nozzle survive from the heat generated by the rocket.
Basically it passes the fuel to the engine through small pipes at the outer layer of the nozzles whereby the fuel is very cold.
Since the fuel is cold, the heat from flame of the rocket will be absorb by the fuel's pipe and avoiding direct heat contact with the nozzle (Which I assume this is what they called Thermal Equilibrium?). Hence avoiding the heat to melt the nozzle.
Answer by @ALAN WARD are far more accurate than mine, but I think the video can provide easier understanding.
Cheers.


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What is meant by empennage height?
I know that the empennage is the tail section of the aircraft and includes the horizontal and vertical stabilisers. But what is the empennage height? Is it the height of the horizontal stabiliser from the ground or is it the height of the vertical stabiliser?

Can you provide more context of this term? Where you found it? Is there any number next to it? etc.

@vasin1987 the original french refers to "tail feathers" e.g. in an arrow but modern usage means "the tail assembly of an airplane".  It is not an obscure term as we borrowed many french words to describe airplane parts.

Of the two possibilities you mentioned, the most likely is the height of the top of the vertical stabilizer from the ground with the plane parked normally. For conventionally-configured tricycle-gear airplanes, this point is the tallest on the aircraft, and therefore this height is useful in determining the required clearance for covered parking areas and hangar openings.
There are a few designs for which a different height measurement is needed. A tail-dragger's tallest point will usually be either the cockpit or a point in the prop's sweep (more likely the cockpit for civilian designs, more likely the prop for old warbirds). The Predator drone uses an inverted V-tail, and its tallest point is actually its communications antenna. The Beechcraft V35 Bonanza's tail tips are within a couple of inches of the top of the cabin. In all these cases a "parked height" figure is required which will be greater than that of the vertical stabilizer.

The empennage height is the distance from the ground to the top of the tailfin.
It determines the minimum clearance the aircraft needs to taxi under an obstruction.


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What general aviation single-engine piston pressurized airplanes are there?
Finding a general aviation single-engine piston airplane that's pressurized is a bit rare. What airplanes that meet these specifications have been produced? Also, which ones are still in production?

The ones I know of

Mooney Mustang M22 (out of production)
Piper Mirage (they claim its the only pressurized piston single on the market)
Cessna P210N (out of production)
Lancair IV-P (falls under experimental but is in "production" none the less)

For what its worth this is a pretty small category of plane to begin with. The M22 was not very successful (although it was ahead of its time) and there are not that many left flying (less than 20 I think). The P210N saw some better success and the Mirage seems to be selling but in the end of the day extra maintenance and complexity of the plane its self does not lend to a very practical plane considering there size and performance numbers.
If we are talking about the less produced or experimental designs we can add,

Junkers Ju 49 - One of the early test beds for pressurization
Farman F.1000 - Altitude record plane
Airco DH.9A - Another early frame used for testing

Also the Lancair IV-P

I was just about to add that!

According to Wikipedia they stopped producing Lancair IV-P already, but their current model, Lancair Evolution is also pressurized and can be powered with piston engine too, though most built so far use a turboprop.


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How is a standard holding pattern flown?
Unless otherwise specified, in what direction and what duration of leg should be flown if assigned a hold?

Although I am not the person who downvoted this question, I agree that it seems to "lack research effort".

Holding procedures are defined by each country for their own airspace. With that said, member countries of the International Civil Avation Organization (ICAO) generally follow the guidance of Part I, Section 6, Chapter 1 in ICAO Doc 8168, Aircraft Operations, Volume I, Flight Procedures:

Section 1.1.3 - The procedures described in this chapter are related
  to right turn holding patterns. For left turn holding patterns, the
  corresponding entry and holding procedures are symmetrical with
  respect to the inbound holding track.

While the document does not explicitly state that right-hand holding patterns are the default, it is implied.

Section 1.4.9 - The still air time for flying the outbound entry
  heading should not exceed: a) one minute if at or below 4 250 m (14
  000 ft); or b) one and one-half minutes if above 4 250 m (14 000 ft).
  Where DME is available, the length of the outbound leg may be
  specified in terms of distance instead of time.

Member countries can deviate from these standards, but if they do, they are supposed to notify ICAO and publish these differences in their respective Aeronautical Information Publication.

Oops, OP totally didn't specify what regs or what altitude. Hah.

Ya, I started writing my answer the same way you did! LOL


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Why do some turboprop engines require motoring after shut down?
A retired pilot I know told me about his time flying a Cessna 441 Conquest II. He said that after shut down he'd have to go out and motor the props. What engines require this procedure and what is the purpose?

I've always heard it referred to as "motoring" conversationally. Maybe we'll get an answer that references the term.

@Airsick I found a manual from a Cessna 208B that uses the word "motoring" several times.

Rotating the propeller of the Cessna 441 Conquest II is to cool the Garrett TPE331 engine in the aircraft. From airliners.net

On the Garrett TPE331's we were trained to spin the props after shutdown to get cooling air flowing through the engine, this was done right after shutdown. Half an hour after we did the prop spins we had go back out to the aircraft and rotate the propellers one blade width. Moving the prop one blade width rotated the engine one half turn, this was to prevent the shaft from sagging as the engine cooled causing turbine blades to rub.

The reason is given as:

Direct drive turbo prop engines sometimes require this to remove residual heat from the engine because once the prop stops turning, so does the engine. When the pilot turns the prop, depending on the gear ratio, he is actually spinning the engine very fast. This helps move some of the heat out of the engine.

Wait... direct drive ... gear ratio ... what?

@egid https://en.wikipedia.org/wiki/Garrett_TPE331#/media/File:Turboprop_cutaway.jpg

Its got an integral gearbox. How is it "direct drive"? :) I don't follow.

@egid, apparently it is direct drive because it has just one turbine (3-stage, but common shaft) that drives both the propeller and the compressor. In contrast most other turboprops have separate HP turbine driving compressor and LP turbine driving the propeller.

The TPE331 in all of its flavors is susceptible to "shaft bow," a condition in which the shaft connecting the compressors and turbines actually bends enough to seize the engine.  Even a tiny amount of bow will seize an engine.  Pilots of planes with the TPE331 (like the Conquest II, Merlins/Metroliner, MU2, etc) are trained to spin the engine through to remove residual heat from the engine core.  Spinning the prop through a few rotation causes the compressor/turbine sections to spin, expelling the heat (...or so pilots are taught!).  With a gear ratio of approximately 23:1, spinning the propeller all the way through once spins the core 23 times.
The primary concern is not heat removal.  No amount of manual spinning will "cool" the engine after it has just shut down!  It's really, really hot inside of a freshly run engine and to think a few rotations of a propeller can exhaust that heat is silly.  Also, as the moment the engine is shut off, it spins freely for hundreds of rotations until it comes to a stop.  That free spinning as the engine winds down is sucking in more cool air than a pilot could possibly move through the engine manually!  The reason pilots are told to spin the prop (whether they know it or not) is to simply reorient the shaft so that it doesn't start to sag.  In reality, after all of that free spinning at shutdown, the propeller only needs to be moved a few blade widths to prevent shaft bow.  At a 23:1 gear ratio, rotating the prop a mere 7.8 degrees reorients the turbine shaft 180 degrees. That measly 7.8 degrees is what really prevents shaft bow!  Operators of TPE331 equipped plane train their pilots to spin props many times just to make sure that the pilots actually move the prop enough to reorient the shaft.  Adjusting the prop 7.8 degrees or spinning it through 1000 times won't hurt the engine...only not rotating the shaft will cause damage...over time...maybe...if you're really unlucky.
Realistically, a pilot exiting a TPE331 equipped aircraft need only give the prop a playful nudge in the normal direction of rotation to prevent shaft bow.  Spinning though X-number of times won't hurt anything, but it's not necessary either.  Also, no need to go out 30 minutes later for another spin session.  These engines aren't made of glass...they just require a moment of attention after they've been run.
A certain company I worked for wanted me to spin the prop through 30 blades after each shutdown.  That procedure did not change between the 3 and 4 blade props...so, exactly why were 30 blades chosen? Never got an answer to that!
The TPE331 is a wonderful engine.


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What is CVG in turbofan engines?
I was chatting with an friend about the Embraer 135/145 and he mentioned that the engines have CVG. I assumed it might stand for "continuously variable" something, maybe "gearing," but I'm not totally sure. What is CVG and how does it impact engine performance?

A bit of Google-Fu: Compressor Variable Geometry. I have no idea what that is however, but maybe this will help you already a tiny bit.

I found an answer in this EM 135/145 manual, but I'd still love an expert answer if someone can elaborate on what it does.

CVG stands for Compressor Variable Geometry. (e.g., this pdf, page 5).
This is a technique to change the pitch angle of the compressor vanes, much like a variable pitch propeller. Wikipedia does not have an article for the use in aircraft (probably because it's not used all that much), but does have an article on a Variable-geometry turbocharger which is closely related (for non-car-enthusiasts: a turbocharger compresses inlet air with help of a turbine at the exhaust). Basically, this system lets you control the pressure ratio independent of the RPM. Advantages include

More efficient settings based on air pressure (altitude / temperature). You basically decouple the turbine stage from the compressor stage (but instead of a gearbox, you use the vane angle). This way, you can optimize your compressor stage to different conditions while keeping the turbine RPM at the most efficient value.
More efficient cruise performance without compromising TO/GA capabilities. Aircraft engines are heavily overdimensioned to account for take-off power and contingencies, which means a bigger engine than strictly necessary and thus less efficiency in cruise. This can now be partially mitigated by varying the compressor vanes pitch for optimal efficiency during cruise and optimal power during TO/GA
Higher engine RPM possible during approach. Normally, to keep your engines spooled up for eventualities, you need to increase their power, and as such need to increase drag with flaps and spoilers to keep the speed down. Now, you can spool up the engine to a high RPM with low compression ratio, and increase power relatively fast without having to overcome engine inertia by increasing the compression ratio by increasing the vane angle.


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Should operations such as landings or approaches be logged when acting as pilot monitoring?
When operating aircraft that require two crew members, generally one pilot acts as PF (pilot flying) and the other as PM (pilot monitoring). How should operations such as landings and approaches be logged when acting as PM?
Is there any guidance on this for pilots operating under the FAA?

I'm assuming that the approach flown is IMC at the FAF.  I'm not saying this is an answer but I don't see the difference between an qualified SIC monitoring an approach and a CFII monitoring an approach.  https://www.faa.gov/about/office_org/headquarters_offices/agc/pol_adjudication/agc200/interpretations/data/interps/2008/ronald%20b.%20levy%20-%20(2008)%20legal%20interpretation.pdf

The Pilot Monitoring (PM) or Second In Command(SIC) cannot log the approach. FAA has answered this in a 1999 letter:

You then ask if, for the purposes of maintaining instrument currency, an instrument approach on the above flight flown by the PIC can be logged as an instrument approach by the SIC.  The answer is no.  As the SIC you have not "performed" the approach as contemplated by FAR 61.57(c) because you were not the sole manipulator of the controls during the approach.

I'm glad they cleared that up with a letter because section 61.57 contains the "sole manipulator" phrase for currency for (a) General experience, (b) night takeoff/landing, (f) Night vision goggles, and the (e)(4) exception for night currency. But it is conspicuously missing from paragraph (c) for instrument experience.


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How long before departure can an IFR clearance be copied?
When flying IFR under the FAA the departure procedure starts with copying an IFR clearance from ATC.
My question is, how long before departure is it okay to go ahead and copy down the clearance?
Sometimes clearances include void times. The context of my question is for clearances delivered, for example, be clearance delivery at a towered airport. These clearances don't include void times. What't the time limit on these clearances?

The short answer is that you may go ahead copy it down as early as they will give it to you!  ;)  The time limit once it has been copied down is a good question.

You kind of have two questions in one - When can I call Clearance Delivery to get my clearance? and How long is that clearance good for?

Casey covered the first part - the local controllers will usually have your clearance 30 minutes before your scheduled departure.
Sometimes you can get this information a little earlier yourself - the new Flight Service tools like 1800WXBRIEF.com can give you expected clearance and flight plan routing information based on what you filed, if the ATC computer already knows what it's going to do with your flight. Here's a few words about it from the Foreflight folks, and you can do the same thing yourself with the Lockheed-Martin tools with a few more manual steps.
You still need to talk to the controllers but if you know what they're going to say it makes that conversation faster!

The second part can go one of two ways: The point of the "void time" is that the ATC system is reserving a spot in the system for you up until that specific time. After that ATC is going to let other aircraft use the airspace they were holding for you (they may allow someone to make an instrument approach to the airport you're leaving from, for example).
This is used a lot for non-towered fields where you have no local controller to talk to.
Clearances from Clearance Delivery at a towered field can include a void time, but they generally don't because they assume you're in the plane and ready to go at or around the time you call in, and you have local controllers to talk to if you have a problem during the run-up or something which delays your departure. Basically the clearance is good until someone tells you otherwise in that situation.
The tool that gets used at towered fields more often is Hold for release: You've been issued a clearance, but you can't execute it until the tower has coordinated you a spot in the airspace. This is generally more efficient than reserving airspace with a clearance and void time when there are controllers to talk to, and if everything is working well your release should be ready right around the time you're ready to take the runway for departure.

You can generally get your clearance 30 minutes prior to the departure time you listed on your flight plan.  If you call clearance delivery prior to that they likely won't have it and will tell you to call back 30 minutes prior to departure.

As casey and voretaq correctly indicated, your flight progress strip containing your route and other information (most pertinently your computer-assigned transponder code) does not become available to the controllers in the tower until 30 minutes prior to your filed "proposed departure" time. You can call sooner than that, but say your callsign very clearly and not super fast—and you might add in something along the lines of "we're a little early." If the controller has a moment, they can edit your P-time and force your strip to print.
The "time limit" on these clearances is set by the overlying ARTCC. Beacon codes are a limited resource and we don't want flight plans holding on to them longer than necessary, so there is a "flight plan drop interval" set for all pending flight plans in that ARTCC's computer. Generally this drop interval is two hours after your P-time—in other words, the window where you can call to copy your clearance is 30 minutes prior to the P-time until 120 minutes after it. During times of long delays in the NAS that drop interval may be increased to three or four hours.
My personal technique is to ensure that any clearance which has been issued to a pilot will not drop out. If the strip marking shows that a clearance has been read, I will keep pushing back the P-time so you don't have to re-file when you're ready. But this is not a guaranteed service, so if you're running late it would behoove you to call again and request that we adjust the time for you.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


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Why do reciprocating aircraft engines have more than one spark plug?
Most reciprocating aircraft engines have two spark plugs per cylinder. What are the reasons for this?

It's not only aircraft engines, the Alfa Romeo's Twin Spark engines have two spark plugs per cylinder as well - mainly for performance reasons.

Bold Method has a great post today about why aircraft engines have more than one spark plug per cylinder. There are a few basic reasons.
Reliability
Having two sparks plugs is more reliable. If one spark plug becomes inoperative for some reason, there's a second to provide the spark for the power stroke.
It should also be noted that the spark plugs are usually powered by dual, independent magnetos or, in some cases, an electronic capacitor discharge ignition. The redundancy of the ignition system adds to the reliability.
More Power
Having two spark plugs means the flame ignites from two points leading to more power per power stroke. This point is demonstrated during run-up. When the magneto check is done the engine RPMs drop when running on a single mag and set of spark plugs.
Even Combustion
Two sparks plugs and a two flame front leads to a more even burn of the fuel air mixture for a smoother running engine.
Prevent Fouling
Aircraft engines burn leaded fuel which can lead to getting lead deposits on the spark plugs. Having dual plugs leads to a more complete burn which can prevent the deposits from forming.

regarding reliability, a plane with dual plugs also has dual magnetos and therefore separate wiring, and the entire ignition system is redundant. So even if an entire mag fails, you still have redundancy.

Good point. Updated.

The "More Power" argument can be demonstrated during run-up, when each magneto is turned off in turn. When running on one mag (and therefore one spark plug per cylinder) the RPM drops.

https://en.wikipedia.org/wiki/Dual_ignition

I'm not sure the more power argument works. In an engine with a single spark plug the plug gap is positioned optimally for best combustion. In a 2 plug system neither plug can be in the optimal position, so having only one plug working will mean less combustion.

@GdD you assume in your comment that there is a single optimal position.  It might also be reasonable to consider that a more even combustion is optimal. The linked (by Digital Trama) wikipedia article demonstrates that (when dual ignition is used in land vehicles where the reliability factor isn't there) efficiency and torque both benefit.

@GdD True, but that already means that the location of the plug matters -  and if one plug on the side is less powerful than one plug in the middle, this also suggests that having a plug on every side is more powerful as well (although obviously not twice as powerful or anything close).

Regarding efficiency due to position of the spark plug, there have been recentish tests of laser based ignition for better fuel burn.

Aircraft engines tend to have a fairly large cylinder capacity, hence a wide bore. The time the flame front takes to travel across this bore is significant. I presume the increased power is due to the most useful section of the fuel burn being biased to the time the crank / connecting rod  is in the most advantageous position. Down side might be that you have a very rapidly increasing combustion chamber pressure (ie, you are burning the mixture significantly quicker with 2 flame fronts) possibly making detonation more likely.

indeed, the 1000CC BMW Boxer airhead motorcycles have a dual-plug upgrade, and there's some very good evidence that it improves performance and has other benefits. http://bmwmotorcycletech.info/dualplugging.htm

@Kickstart: Why not have three or four or six sparkplugs per cylinder, then, rather than just two?

@Sean - I suspect no further advantages, added to which adding more limits other parts of the design (such as valve position - easy enough to have 2 spark plugs of a 2 valve per cylinder engine). Aircraft engines tend to be low revving, so a reasonable amount of time for the mixture to burn, while more spark plugs = faster rise in cylinder pressure.

Engine Longevity as well. The more even combustion resulting from dual sparks prevents hot spots inside the cylinder, resulting in improved engine life.

Reliability is the key thing. The ignition system of a gasoline engine (the "magnetos" in most aircraft) is absolutely critical to the engine's operation, and thus the safety of the flight of the aircraft possessing it. A small break in a wire, or fouling of one plug, could cause cylinder misfires or even a total engine failure. To guard against this, dual independent ignition systems are incorporated into aviation engines to allow for continued operation in the event that any component of one ignition system fails.
Efficiency is a secondary benefit. Two spark plugs per cylinder means the spark initiates combustion at two points within the volume of the cylinder. Combustion rate of a vaporized fuel is typically a function of "surface area" of the region of combustion, so two sparks equals double the combustion surface area. This produces faster and more complete combustion, increasing power at a given fuel flow rate. One of the rudimentary "runup" tests of a piston engine involves selecting one or the other magneto, and watching the performance of the engine. The normal result with a healthy ignition system is a slight reduction in power, as indicated (in a fixed-pitch prop at least) by lower RPM. A severe power reduction, rough operation or an engine quit is a sign something's wrong on that side of the ignition system.

How would a failure of one sparkplug or the breakage of one sparkplug wire be capable of causing a total engine failure?  One would expect, at the very worst, that particular cylinder to stop producing power, but the rest of the engine to keep burning happily.

@Sean - depends on which wire breaks, as he didn't mentioned sparkplug wires. One inside the magneto could easily stop all sparks on that system. And in certain circumstances it would be possible for a failed spark to result in enough fuel accumulating in a cylinder rapidly to hydraulic the engine (this can be an issue with high performance engines running on methanol)


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How is "information relating to the safety of flight" defined by the FAA?
14 CFR 91.183 states:

Unless otherwise authorized by ATC, the pilot in command of each aircraft operated under IFR must report the following as soon as possible: Any unforecast weather conditions encountered; and any other information relating to the safety of flight.

This language is very vague and probably causes a lot of confusion for pilots who are studying it. Does the FAA elaborate on this concept or provide an examples or a more specific definition somewhere else? If not, what are some examples of reports ATC would expect based on this reg?

The language is intentionally vague.
Much like Justice Stewart said about pornography, when it comes to "information relating to the safety of flight" you'll know it when you see it.

Birds flying around in the approach path? Yup.
Wind shear on short final? Yup.
(Even if it's forecast/reported telling them how much airspeed you gained or lost helps.)
"Drone" buzzing around in the traffic pattern? Yup.
Deer grazing in the middle of a grass runway? Yup.
Bird strike? Absolutely.
(There are regs/procedures specifically for this.)
The guy in row 3J is threatening the flight attendants? Yup.
The guy in row 3J spilled his soda on his lap? …Not so much.

Ha I love this answer! Thank you! Also it helped me realize that it's not vague because of lawyer bullcrap (well maybe a little), but it's vague because reporting information that affect the safety of flight can make things safer for everyone. Also what's the reg that specifically requires reporting of a bird strike?

@RyanBurnette From a regulatory perspective I don't think there's a specific rule for wildlife strikes (though they can be swept up in things that cause "substantial damage" depending on where and how the bird hits the aircraft, and I think there may be something in Part 139 for airport operators). The FAA's Wildlife Strike Database is mostly bird-related incidents though, and theres a tidy little mini-bureaucracy for dealing with that stuff.

tidy little mini-bureaucracy There is nothing tidy, little or mini about bureaucracy. That was, however, a nice turn of phrase. ;)

@RyanBurnette, it's vague most of all because real life is complicated and there is simply no chance to create an even remotely complete list off the table.


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What do numbers signify in airport codes?
As an Air Cadet, we go gliding very frequently. However, I've noticed something about the airport that we go to. The airport that we go to, Markham Airport, has a different airport code. CNU8.
Meanwhile, other airports, such as Toronto-Buttonville, Toronto-City-Centre, and Toronto-Pearson all have the following codes respectively: CYKZ, CYTZ, and CYYZ.
Do the numbers in airport codes signify something? If they don't, why is there a mix of both letters and numbers?

The code for Markham Airport, CNU8, is a Transport Canada identifier.
The codes for the other airports are ICAO airport codes. ICAO airport codes always consist of 4 letters. Markham Airport does not have an ICAO airport code.

Interesting. I didn't catch that. Is this just a Canadian thing, or do similar things apply worldwide?

In many countries there are small airfields that do not have an ICAO or IATA identifier. Mostly these are glider or helicopter fields. Often there is a local identification scheme for coding these airports.

@DeltaLima -- in the US at least, many of them service fixed-wing GA birds as well

Yes, lots of GA airports in the U.S. don't have ICAO codes. However, that Wiki article seems to suggest that ICAO codes can contain numbers: "The ICAO airport code or location indicator is a four-character alphanumeric code."

The FAA's rules are in their Location Identifiers paper, which explains how the numbers are used, at least for the US. It also mentions Transport Canada, by the way, and I guess that Canada uses similar rules.
There are specific requirements for an airport to qualify for a three-letter identifier:

a. Three-letter identifiers are assigned as radio call signs to
  aeronautical navigation aids; to airports with a manned air traffic
  control facility or navigational aid within airport boundary; to
  airports that receive scheduled route air carrier or military airlift
  service, and to airports designated by the U.S. Customs Service as
  Airports of Entry. Some of these identifiers are assigned to certain
  staffed aviation weather reporting stations or for airports
  commissioning Automated Weather Observation Systems, level III
  (AWOS-III) or higher that have paved runways 5,000 ft or longer.

That three-letter code is then prefixed with 'K' for the continental US, to give the four-letter international code:

An international location indicator is a four-letter code used in
  international telecommunications. The location indicator for airports
  in the contiguous United States is the three-letter identifier
  preceded by "K". For other non-contiguous United States airports, the
  following two letter prefix will be used:
Alaska - PA, PF, PO, PP
  Hawaii - PH
  Puerto Rico - TJ
  Virgin Islands - TI
  (See ICAO Document 7910 for listings.)

The one-letter two-number format is usually used for public airports that don't qualify for a three-letter identifier:

d. Most one-letter, two-number identifiers are assigned to public-use
  landing facilities within the United States and its jurisdictions,
  which do not meet the requirements for identifiers in the three-letter
  series. Some of these identifiers are also assigned to aviation
  weather reporting stations.

One-letter, two-number identifiers are keyed by the alphabetical letter. The letter may appear in the first, middle or last position in
  the combination of three characters. When the letter signifies an Air
  Traffic Control Center's area, the assignment will not change if the
  Center's boundaries are realigned.

The two-letter, two-number format is usually used for private airports that don't qualify for a three-letter identifier:

e. Two-letter, two-number identifiers are assigned to private-use
  landing facilities in the United States and its jurisdictions which do
  not meet the requirements for three-character assignments. They are
  keyed by the two-letter Post Office or supplemental abbreviation
  (listed below) of the state with which they are associated. The
  two-letter code appears in the first two, middle, or last two
  positions of the four-character code.

There are numerous coding schemes for airport identifiers... they're often broken down into 3 main categories. ICAO, IATA, and country of origin. Sometimes these are all the same, other times, each one is completely different from the other. Each of these schemes evolved for different purposes(or diverged from each other at various times, aka ICAO added a fourth character from the IATA code bank and then diverged from there).

ICAO - International air traffic
IATA - Ticketing information for the airlines.
Country of origin - A way to identify all the airports in a specific country.

For example, in the US, there are some 15,000 airports. Most are small grass strips or similar, but each often needs its own identifier. There aren't enough 3 letter or 4 letter only combinations, so they start adding numbers. 4V4, 20GE, etc.

"sometimes these are all the same"—ICAO and IATA codes can't ever be the same, because ICAO codes are 4-letter and IATA ones are 3-letter.

@JanHudec I think by "the same" he means the last three letters are the same, which is often true in the USA and Canada.

@Random832: Yes, in US and Canada the ICAO code is often the IATA code with K (in US) or C (in Canada) prefix. In the rest of the world it is rare, because most ICAO location prefixes are 2-letter.


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Is there a boomless sonic technology in any state at this time?
I've heard rumors about airplanes being on the horizon with "boomless sonic" technology, or the ability to fly supersonic without creating that hazardous shockwave.
This would be very valuable for business travelers who want to cover populated areas at high speed. These aircraft would presumably be certified to fly at supersonic speeds where it is now forbidden due to the damage of sonic booms.
Does this technology exist? Is anyone on the trail to making this work? Is it even possible?

Shock wave is a physical phenomenon and as such is unavoidable. The best we can do is to reduce their intensity and reduce their effects on ground.
A method to eliminate the shock wave (theoretically) was proposed by Adolf Busemann. His airframe, called the Busemann's Biplane which consists of two triangular cross-section plates a certain distance apart (so that choking is prevented) with the flat sides parallel to the fluid flow.

Image from Supersonic biplane—A review by Kazuhiro Kusunose et al.
The idea is that the shock waves are formed internally between the two (triangular) 'airfoils' and cancel each other out at design condition, shown in the first case.

Image from Supersonic biplane—A review by Kazuhiro Kusunose et al.
At off design conditions, shock waves are produced. However, this doesn't produce any lift at zero angle of attack, as is obvious. As researchers explain,

... the design lacks lift: The two wings create a very narrow channel through which only a limited amount of air can flow. When transitioning to supersonic speeds, the channel ... could essentially “choke,” creating incredible drag. While the design could work beautifully at supersonic speeds, it can’t overcome the drag to reach those speeds.

Research is being carried out to overcome this issue- One method proposed is to have the wings 'morph' shapes- so that they are efficient at all speeds. Another group is working to optimize the shape of the airfoils in Busemann's Biplane so that the shock-wave is reduced at all stages of supersonic flight. However, all these studies are in conceptual stage and no experiments have been carried out so far.
NASA carried out the Shaped Sonic Boom Demonstration(SSBD), in which the fuselage of a Northrop -5E was modified (kind of like pelican's beak) so as to change the pattern of shock waves formed in supersonic flight.

"F-5E Shaped Sonic Boom Demonstration aircraft" by NASA/Carla Thomas - http://www.dfrc.nasa.gov/Gallery/Photo/SSBD/HTML/EC03-0210-1.html. Licensed under Public Domain via Commons.
NASA reported that the modifications were able to reduce the intensity of the shock wave by a considerable amount, as shown in the image below (the 'flat' pressure line is from the SSB demonstrator).

Image from Quieting the Boom- The Shaped Sonic Boom demonstrator and the Quest for Quiet Supersonic Flight by NASA
Active research is still being carried out by various organizations in this field.

Yes, there is this technology already. By using the fact that the speed of sound is proportional to the square root of the air temperature, you can fly at a low supersonic speed at altitude and no shock wave would hit the ground. If the Mach number is low enough, your speed will be subsonic in the air at ground temperature, so the shock waves will dissipate on their way down.

Graphical explanation of the threshold Mach number (picture source)
But the speed gain would be low and the whole endeavor not very economical. Once the flight speed is increased such that it is also supersonic at ground conditions, audible shock waves will be hard to avoid. They can be mitigated, but not completely avoided.

Now, these are really just a collection of stuff I have read and I am not an expert and I am writing these from vague recollection, so errors can be expected. Corrections welcome. Still thought it makes sense for me to write these, as it seems to me this type of concepts is what the question was about. These are what is usually meant when "boomless supersonic" is talked about.
Simplest way is to fly high. Lower air density means lower energy density in the boom. And greater distance means more energy will be dissipated before ground. People are always making concepts about sub-orbital shuttles that would be some where between Concorde and a ballistic missile. Would generally require a new type of engine to be practical.
It is possible to attach a boom ahead of the fuselage which will have its own smaller sonic boom. This will reduce the intensity of the sonic boom of the fuselage and the sum would be smaller than without the boom.
You can use the sonic boom and thus absorb a part of its energy.
You can use it to create compression for the engines. Basically you would trap the shock wave with the engine intakes. There would be drag and hard to imagine the same engines working well subsonic. And it would be hard to combine with generating useful lift. Also while using compression from the fuselage is normal, actually capturing the sonic boom would require the intakes to be close to the front, which would cause problems with engine design.
You can use the energy of the shock wave to generate lift. More lift you generate, less will be left to annoy people on the ground. Basically you shape fuselage to generate more of the shock to the sides and attach to the wings a surface at an angle that reflect the shock wave downwards. Reflecting energy down will create lift. You can also have a fuselage that directly generates downwards shock waves, but in several steps for greater lift and less noise. A downside of such waverider concepts is that they only really work in supersonic, I think even a specific range of supersonic. So until you reach correct velocity you'll only have more drag.

I recently saw something like this in a document about a supersonic concept plane (by boeing or airbus, I don't remember)
The main part here was that it isn't really the airplane that is "boomless" but rather that it is capable of flying a "boomless" (note the quotes i'll come back to it) flight profile.
The "boomless" profile they described incuded transitioning to supersonic in near vertical flight (under the power of rockets/ramjets), spreading the shockwave in the horizontal plane, giving it time enough to dissipate energy before it comes near the ground.
The cruise altitude of this plane was of sufficient hight to prevent boom issues on the ground (enough hight so that the shock wave would be dissipated by the time it reaches the ground)
So basically it comes down to a near vertical climb and high altitude flight.

Somehow I doubt this will be popular in commercial passenger operations. Six Flags Airlines might be interested, though.

I guess it's not an airliner, but a military fighter. That would rule out Airbus, they don't do fighter planes.

@Alexander: afaik it was an airliner, and it was airbus, but aparently it's just a patent of a concept
https://www.youtube.com/watch?v=0dyrbRGkVKI


text:
What is exemption 17347 in the context of dispatch for FAA domestic carriers?
What is exemption 17347 in the context of dispatch for FAA domestic carriers?
The explanation can be rather complicated, so I’m looking for a simple and straight forward break down.
Also, as a bonus, what is the history behind this exemption?

Exemption 17347 was requested by the Regional Airline Association to extend a pre-existing alternate weather regulatory exemption to apply to regional airlines operating under part 121 because the FAA has changed the language referring to other carriers used in grants of exemption.

As a separate issue, when A4A previously petitioned for this exemption, the FAA granted relief to the petitioner’s member airlines, as well as to all other “similarly situated” certificate holders, which included any air carrier operating under the provisions of part 121 (Exemption No. 3585). However, in an effort to manage exemptions granted by the FAA and to facilitate operator compliance with the conditions and limitations of a grant of exemption, the FAA is no longer including “similarly situated certificate holders” in grants of exemption.

Regulatory Filing Source


text:
What is the fastest single-engine civil turboprop aircraft with cabin seating?
I was having a discussion with some friends about general aviation aircraft with cabin-class seating. We were discussing cruise speeds and there were some "shop talk opinions" flying around about what the fastest single-engine turboprop in general aviation is. I'm curious to know who holds the title.

Fastest single-engine prop? P-51 Mustang, of course! Oh, turbo prop, GA, cabin seating. sigh...

The TBM 900 holds the title of fastest single engine turboprop for civilian planes. It has a top cruise speed of 330 knots at FL280.

In civil GA aircraft, TBM 900 is the fastest one with a top cruise speed of 330 knots (at 28000 ft).

Source: flightclub.jalopnik.com
However, if one considers all single engined turboprop aircraft, the fastest is the Pilatus PC-21, with a top speed of 370 knots.

"Pilatus pc-21 hb-hzc lands arp" by Adrian Pingstone (Arpingstone) - Own work. Licensed under Public Domain via Commons.

A one-off homebuilt stole the title from the PC21, the 850SHP two seat modified Lancair Legacy "Turbulence" cruises around 375-380 knots.

Is this plane currently sitting at KPAO in California? I swear to god it was tied down next to my trainer when I was getting my license.

The Lancair Evolution comes to mind coming in at 300Kts

(source)
The Pilatus PC-21 comes in at 370Kts but those are mainly military training planes so it may not fall under the GA space.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

authors: Андрей Еловский


text:
Why does maneuvering speed vary with weight?
Aircraft certified by the FAA under 14 CFR Part 23 have many V speeds, one of them which manufacturers are required to determine and publish in the POH is maneuvering speed, or Va.

In aviation, the maneuvering speed (Va) of an aircraft is an airspeed limitation selected by the designer of the aircraft. At speeds close to, and faster than, the maneuvering speed, full deflection of any flight control surface should not be attempted because of the risk of damage to the aircraft structure.

This speed varies with the weight of the aircraft. Why does this speed vary with weight? Additionally since manufacturers often only provide that value at max gross weight, how does one figure out what that Va airspeed is when not at max gross weight?

Compute maneuvering speed below max gross using the formula $V_A\sqrt{\frac{W_2}{W_1}}$, where $V_A$ is the maneuvering speed at max gross, $W_2$ is actual weight, and $W_1$ is max gross.
We can derive this relationship — or for any other V-speed such as stall speed of landing speed that varies with weight — from the lift equation. In steady-state flight, weight equals lift so
$$W_1 = \frac{1}{2} C_L \rho v_1^2 S$$
and likewise for $W_2$ and $v_2$. Dividing the first by the second cancels the coefficients and leaves
$$\frac{W_1}{W_2} = \frac{v_1^2}{v_2^2}$$
Take the square root of both sides and solve for $v_2$ to arrive at the general formula
$$v_2 = v_1 \sqrt{\frac{W_2}{W_1}}$$
John Denker provides an intuition for why the relationship works the way it does.

Unlike $V_{NO}$, the maneuvering speed varies in proportion to the square root of the mass of the airplane. The reason for this is a bit tricky. The trick is that $V_A$ is not a force limit but rather an acceleration limit. When the manufacturers determine a value for $V_A$, they are not worried about breaking the wing, but are worried about breaking other important parts of the airplane, such as the engine mounts. These items don’t directly care how much force the wing is producing; they just care about the acceleration they are undergoing.
By increasing the mass of the airplane, you decrease the overall acceleration that results from any overall force. (Of course, if you increase the mass of cargo, it increases the stress on the cargo-compartment floor — but it decreases the stress on unrelated components such as engine mounts, because the acceleration is less.)

Later in the same section, Denker clarifies.

Finally, we should note that there are two different concepts that, loosely speaking, are called maneuvering speeds.

The design maneuvering speed, which we can denote $V_{A(D)}$, is primarily of interest to aircraft designers, not pilots. The designer must choose a value for $V_{A(D)}$ and then build an aircraft strong enough to withstand certain stressful maneuvers at that speed. Higher values of $V_{A(D)}$ promote safety, by forcing the design to be stronger.
The maneuvering speed limitation, which we can denote $V_{A(L)}$, is of interest to pilots. It is an operating limitation. It appears on a placard in the cockpit. Lower values of $V_{A(L)}$ promote safety, by restricting certain operations to lower, less-stressful airspeeds.

Denker, John S., See How It Flies, §2.14.2 “Maneuvering Speed,” accessed 16 Aug 2015.
The discussion above pertains to the maneuvering speed limitation, i.e., values a pilot would find in a POH or on placards. For example, the humble Cessna 152 POH shows $V_A$ decreasing with decreasing weight: 104 KIAS at the max gross of 1,670 pounds, 98 KIAS at 1,500 pounds, and 93 KIAS at 1,350 pounds. The reader will also note that these values fit the general relationship given at the beginning of this answer.

@RyanBurnette: No, you have it backwards. $v_A$ is defined with stall speed and maximum load factor, and then all parts need to be made strong enough that they don't break off when maneuvering at this speed. $v_A$ is only a design speed - some speed needs to be defined, and that is the definition for one load case of the vertical. What is not covered is building up of sideslip with several rudder inputs and then adding the load of sideslip and full deflection, however, a good design will also cover this case (it would be easier to calculate, anyway).

"So you say that Va goes up when the aircraft mass goes up? This is certainly not true." Quoting from Cessna 152 1980 POH: Va: 1350 Pounds -> 93 KIAS, 1500 Pounds -> 98 KIAS, 1670 Pounds -> 104 KIAS.

I have often wondered whether in some a/c like gliders, the concept of Va going up as aircraft weight increases is not really appropriate.  I understand why that makes sense in some a/c where Va is an acceleratn limit, but if the extra weight is being added exclusively to the fuselge in the form of pilots sitting in strong seats, and there's not a lot of stuff bolted on like engines and batteries, it seems in some aircraft perhaps Va ought to go down as weight goes up, because the mass of the wings absorbs proportionately less of the total lift force, so there is more strain on the wing bolts.

@PeterKämpf I believe you are thinking of the design side and are far more qualified to discuss that aspect. This answer shows how to determine from more-or-less design value the different limitation values at varying weights.

@GregBacon: I get what John Denker says, but in order to limit accelerations, you limit accelerations (maximum and minimum gs), not speed. Again, this is getting it backwards. What he means is the speed for maximum control deflections, and - given a high enough v$_A$ during design - this will be lower in order to limit accelerations. But the better procedure is to limit those accelerations directly.

The maneuvering airspeed $v_A$ ensures that the maximum structural load is not exceeded even with maximum control surface deflection. Wikipedia says this is valid only when a single control surface is maximally deflected, but really the regulations try to make sure that any combination of single control inputs is safe. To determine the structural load, one must know the mass of all non-lift-creating parts. Fuel in the wing tanks does not count, as it is carried by the lift created right around it and does not increase the wing root bending moment.
Given: A maximum wing root bending moment $M_{b_{max}}$, a maximum load factor $n_{z_{max}}$, a maximum lift coefficient $c_{L_{max}}$ and a gross mass of all parts supported by the wing (fuselage, payload, fuselage-mounted engine, …) of $m_{nlc}$. Further, let's assume that the center of lift of one wing with the area $½S$ is out at a wing station $y_L$ (to be precise, use the center of lift with maximum down aileron deflection, measured from the attachment point of the wing at the fuselage), and the mass of the wings (plus fuel in the wing tanks and engines on the wing) is $m_{total} - m_{nlc}$. The lift of one wing is
$$L = ½S\cdot\rho\cdot ½v^2\cdot c_{L_{max}}$$
and the root bending moment is
$$M_b = \left(L - ½(m_{total} - m_{nlc})\cdot n_z\right)\cdot y_L$$
Note that the bending moment ist not just lift times lever arm, but is reduced by the part of the airplane's mass which is contained in the lift-creating parts, all calculated for one side!
The maximum allowable speed at which this bending moment reaches the maximum allowed value is
$$v_A = \sqrt{\frac{4\cdot\frac{M_{b_{max}}}{y_L}+2\cdot(m_{total} - m_{nlc})\cdot n_{z_{max}}}{S\cdot\rho\cdot c_{L_{max}}}}$$
Now you need only to know how the total flight mass $m_{total}$ of your aircraft is divided between what is carried at the wing root and what is the wing and attached to it. Increasing the payload carried in the fuselage, or adding fancy equipment in the cockpit will reduce $v_A$, and increasing fuel in the wing tanks will have no effect. If you add mass outside of $y_L$ (like with tip tanks), the bending load is decreased and $v_A$ goes up.
Note that conditions exist where the maximum tolerable structural loads can be exceeded even at $v_A$: If the pilot moves one control surface repeatedly with the eigenfrequency of a structural or a rigid-body eigenmode, the aircraft can build up angles of attack beyond those achievable with a single input. Consequently, the stresses can grow above those used for sizing the structure.

"Increasing the payload carried in the fuselage, or adding fancy equipment in the cockpit will reduce Va."
Ah, now that's the answer the mathematically illiterate among us were looking for.

It's not true that Va ensures maximum structural load no matter what maneuver is flown. It only provides that assurance if a single control surface is fully deflected in one direction. See the other answer's reference of American Airlines 587 and the resulting CFR release.

@RyanBurnette: Technically, it needs two $v_A$s, one for longitudinal and one for lateral maneuvers. Also, the rate of flow angle change is important, as is the excitation of eigenfrequencies by a sequence of control inputs. This topic is much more complex than my answer shows, but I wanted to show the basic thinking behind the determination of $v_A$.

@PeterKämpf I think you should edit the first sentence. Propagating this myth is could create a dangerous condition for pilots who operate under this misconception.

@RyanBurnette: Wikipedia calls it a myth, and Airbus got away with this interpretation that $v_A$ is only for single control surface deflections. But the idea is really that you can put the aircraft in any attitude with all control surfaces, and still parts don't break off. Take an accelerated stall: You fly a coordinated turn an pull until the maximum lift is reached. You use all control surfaces and still the aircraft must not break apart. You may even add sideslip - still the structure must stay intact. Wikipedia creates a myth of its own here!

From AC 23-19A: "Va should not be interpreted as a speed that would permit the pilot unrestricted flight-control movement without exceeding airplane structural limits."

More importantly from the summary of the resulting CFR Final Rule: "The Federal Aviation Administration amends the airworthiness standards applicable to transport category airplanes to clarify that flying at or below the design maneuvering speed does not allow a pilot to make multiple large control inputs in one airplane axis or single full control inputs in more than one airplane axis at a time without endangering the airplane's structure."

Granted the final rule applies to aircraft certified under 14 CFR 25, not 23, I still think Va needs a rewrite in a lot of training materials.

At the normal stall speeds given by $V_s$, the load on the aircraft is 1g, and the lift is equal to weight. i.e $L = W$.
In case of maneuvering, the load factor is greater than one, and we have $L = nW$, with $n$ being the load factor.
We have,  $L = nW = \frac{1}{2}  C_L  \rho  V^2 S$.
This gives maneuvering speed , $V_a$ = $V_s \sqrt{n}$
It can also be written as $V_s$ = $\sqrt{\frac{2 n W}{\rho C_{Lmax} S}}$
At maximum weight, this gives,
$V_A$ = $\sqrt{\frac{2 n W_{max}}{\rho C_Lmax S}}$
For other weights, we have,
$V_a$ = $\sqrt{\frac{2 n W_{a}}{\rho C_Lmax S}}$
For the same load factor, we then have,
$V_{a} = V_{A} \sqrt{\frac{W_{a}}{W_{max}}}$,
where
$V_{a}$ is the maneuvering speed at weight $W_{a}$ and $V_{A}$ is the maneuvering speed at maximum weight $W_{max}$
As the maneuvering speed depends on the load factor and the stall speed, it depends on the weight of the aircraft (which decides the stall speed). It is basically a structural limit.
Another thing to note is that the aircraft can sustain structural failure even below maneuvering speed when multiple large control inputs are given.

This is generally right - but note that $V_A$ can be be defined as anything from $V_S\sqrt{n}$ to $V_C$ - cruise speed.

... So I went and watched the tutorial.  Turns out Va is a case where STALLING is a built in SAFETY feature.  Va is a speed limit, above where the airplane will exceed its G load limit before it stalls if abruptly manuvered.
How does weight factor in? A heavier plane flies at a higher AOA to generate adequate lift at a given speed, or the same AOA at a higher speed.
The universal AOA formula for the plane would be stall AOA/Va AOA is less than or equal to G load limit (Accounting for linearity of the lift vs AOA curve).
The heavier plane would need higher speed than a lighter plane to reach Va AOA limit in straight and level flight.  Too small an AOA sets you up for exceeding G load limits in an event such as severe turbulence.
Properly working AOA sensors are valuable instruments for this application, as well as the airspeed indicator.

@Peter Kampf From your engineering point of view, let's design a Vstab/rudder.  Should IT stall before "blow down"?  Yes, here larger mass plays a role, as a full rudder deflection will stress a heavier weight more ( it's acceleration will be less) - it breaks, rather than "gives" or "rolls with the punch".  Graphicly, you will see a higher peak load for a longer period of time. For placard Va, seems like they are referring more to departure from cruising AOA.

So, for the Vstab/rudder, a brisk control application would put peak load at or near Tzero, the lighter one "gives" faster to reach "zero lift" zero load state (opposite forces rebalance).


text:
What qualifications are required to become chief instructor at a 141 flight school (FAA)?
Under the FAA jurisdiction flight school operations can be conducted under the provisions of part 61 or part 141. If a flight instructor wanted to become a chief instructor at a part 141 flight school, what would they need to be qualified?

The requirements are in 14 CFR 141.35, Chief instructor qualifications. They're too long to quote here in full and the exact requirements depend on the type of instruction, but in summary:

Hold a commercial or ATP certificate
Hold a CFI certificate
Pass knowledge and proficiency tests
Meet minimum flight time requirements (depending on the type of instruction, e.g. 1000 hrs PIC time is required for private pilot instruction)
Meet minimum flight training time requirements

For training pilots for an instrument rating, the chief instructor also needs an instrument rating, instrument flight time and instrument training experience.


text:
What visibility and cloud clearances are required for airplanes conducting visual approaches?
What visibility and cloud clearances are required for airplanes conducting visual approaches during IFR while flying under FAA regulations? Is the visual approach a purely VFR maneuver requiring the usual VFR weather minimums for the airspace being flown? Or does the visual approach maneuver flown under IFR supersede normal VFR weather minimums?

This depends:
Under part 91, you just have to remain clear of clouds and have a minimum of a 1,000 ft. ceiling and 3 SM visibility.
Commercial operations (121 and 135) are further restricted by their Operations Specifications and have to maintain VFR weather minimums appropriate for the airspace that they are in (among other things, see below).
Here is an excerpt from the AIM (there is more details there for different operations, with the differences between controlled and uncontrolled fields, etc.)

5−4−23(a). Visual Approach
a. A visual approach is conducted on an IFR flight plan and authorizes a pilot to proceed visually and clear of clouds to the
  airport. The pilot must have either the airport or the preceding
  identified aircraft in sight. This approach must be authorized and
  controlled by the appropriate air traffic control facility. Reported
  weather at the airport must have a ceiling at or above 1,000 feet and
  visibility 3 miles or greater. ATC may authorize this type approach
  when it will be operationally beneficial. Visual approaches are an IFR
  procedure conducted under IFR in visual meteorological conditions.
  Cloud clearance requirements of 14 CFR Section 91.155 are not
  applicable, unless required by operation specification.

The Part 135 Ops Spec for this contains the following requirements for a visual approach:

b. Terminal arrival IFR - Visual approach or a Charted Visual Flight
  Procedure (CVFP).
  The flightcrew may accept a visual approach or a
  CVFP provided all the following conditions exist. The flightcrew may
  not accept a visual approach or a CVFP unless the limitations and
  provisions of subparagraph f. of this operations specification are
  met.
(1) The flight is operated and remains in Class B, C, or D
  airspace, within 35 miles of the destination airport in Class E
  airspace, or the airspace beneath the designated transition area.
(2)
  The flight is under the control of an Air Traffic Control (ATC)
  facility.
(3) The flightcrew must maintain the basic cloud clearance
  as specified in Section 91.155.
(4) For a visual approach without a
  CVFP - The flightcrew must be able to establish and maintain visual
  contact with the airport or maintain visual contact with the traffic
  to be followed as directed by ATC. In addition, the following
  provisions and weather conditions at the airport during the approach
  must be met:
(a) Reported visibility must be as specified in Section
  91.155, but not lower than a visibility of three miles and reported ceiling must be 1,000 feet or greater, or
(b) When in the terminal
  area with the reported visibility not lower than three miles and
  ceiling not reported, the flightcrew may continue to a landing if the
  runway of intended landing is in sight and the flightcrew can maintain
  visual contact with the runway throughout the approach and landing,
  and
(c) Ceiling and cloud clearance must be as such to allow the
  flightcrew to maintain the minimum altitudes prescribed in Section
  91.129, 91.130, or 91.131, as applicable for the airspace class in which the flight is operated.
(5) For a CVFP - The flightcrew must be
  able to establish and maintain visual contact with the airport or the
  charted visual landmark(s) for the CVFP throughout the approach and
  landing. In addition, the weather conditions at the airport at the
  time of the approach must be reported to be at or above the weather
  minima established for the CVFP, but never lower than the VFR landing
  weather minima stated in Section 135.205 in uncontrolled airspace.
...
f. Special Limitations and Provisions for Visual Flight Rules. All VFR operations authorized by this operations specification shall be
  conducted in accordance with the following limitations and provisions.
(1) The certificate holder must identify obstacles and use airport
  obstacle data which ensures that the performance requirements of Part
  135 are met.
(2) The weather conditions must allow the flightcrew sufficient
  visibility to identify and avoid obstacles and safely maneuver using
  external visual references and to maintain minimum altitudes.

What visibility and cloud clearances are required?
VFR, as per 91.129, 91.130, or 91.131 for the various classes of airspace

Can you expand your answer a bit? The mention of VFR sounds like the required weather for the conditions, but the required cloud clearance can vary.

I see that you updated your answer. This seems to contradict the other answer. You're saying that VFR weather minimums for the particular airspace must be respected?


text:
Why do some approaches have suffix letters such as Y and Z?
There are cases with instrument approaches where there will be multiple approaches to the same runway, in many cases uses some of the same nav aids and positions, but differentiated using alpha characters.
One example are the ILS Y or LOC Y RWY3 and ILS Z or LOC Z RWY3 approaches at Raleigh Exec Jetport at Sanford-Lee County in Sanford, North Carolina. These approaches both use the LOC/DME I-TTA there, and some of the same positions.
It seems that the differences are subtle in the overall procedure on things like the missed approach procedure and how positions are identified, but I'd love a textbook answer that explains it more thoroughly.

ILS Y and Z (click either image for higher resolution).

In this case it seems the difference is the minimum altitude at AMIRS (due to the surrounding obstacles). Y approach which requires GPS and radar contact has lower minimums.

The naming convention is explained in Chapter 4 of the FAA's Instrument Procedures Handbook:

When two or more straight-in approaches with the same  type of
  guidance exist for a runway, a letter suffix is added  to the title of
  the approach so that it can be more easily  identified. These approach
  charts start with the letter Z  and continue in reverse alphabetical
  order.

The TERPS explanation (section 161) is a little more specific:

d. Duplicate identification suffix. When more than one procedure to the same runway uses the same type of navigation system for lateral
  guidance within the final approach segment, differentiate each
  procedure by adding a non-repeating alphabetical suffix using the
  letters “S” through “Z.”

The Handbook gives the example of two approaches at Chicago Midway (KMDW), which has the RNAV (GPS) Z RWY 13C and RNAV (RNP) Y RWY 13C approaches. They're both straight-in GPS approaches to the same runway, but one requires RNP and the other doesn't.
As the question itself and this comment note, there are plenty of other possible differences between approaches to the same runway; RNP is just one example. Having multiple approaches allows an airport to accommodate aircraft with different equipment and capabilities or, as section 140 of the TERPS puts it, to "gain an operational advantage":

To permit use by aircraft with limited navigational equipment, an IFP [Instrument Flight Procedure]
  should be formulated using a single navigation source whenever
  possible. The use of multiple navigation sources of the same or
  different types may be permitted to gain an operational advantage

I have no idea how to find out why a specific runway has multiple approaches or how the approaches were designed; presumably it's something that the FAA and the airport operator (and maybe even the airlines) work out based on local runway use and traffic requirements.
In the example in the question, one plate requires GPS, the other DME or radar. So a non-GPS equipped aircraft can still use the ILS, but with different ways of getting to it, different minima, etc.
Note that there's a similar naming convention for circling approaches:

Approaches that do not have straight-in landing minimums  are
  identified by the type of approach followed by a letter.  [...] As can
  be seen from the example, the first approach of  this type created at
  the airport is labeled with the letter A,  and the lettering continues
  in alphabetical order.

Their example is Rogue Valley International - Medford (KMFR), which has the following circling approaches:

VOR-A
LOC/DME BC-B
VOR/DME-C
RNAV (GPS)-D

There's no explanation that I could find in the Handbook or TERPS to explain why circling approaches must have a letter even if the navigation systems are different. But as @RalphJ mentioned in a comment, this provides a clear way to identify multiple circling approaches using the same guidance, e.g. a VOR-A approach from the north and a VOR-B one from the south.

Another reason you might have a "Y" and a "Z" approach could be the use of different navaids to define fixes on the approach or missed approach.  For instance, one version of the approach might define the FAF based on an NDB, while another version defines it as a DME fix.  An aircraft equipped with one but not the other could fly whichever version it could use.  Alternatively, missed approach holding or holding-in-lieu-of: holding at a VORTAC works if equipped with a VOR, but can not be done if only TACAN equipped.

While this says when they use multiple letters, it doesn't really explain why which is the question (other than the RNP / non-RNP example, which doesn't apply to the situation they present).

@Lnafziger Isn't that exactly what the quote explains? If there are multiple approaches to the same runway that use the same guidance method then there needs to be some way to tell them apart.

As to 'why circling (only) approaches must have a letter,' it's because they don't have a runway ("VOR22") and they need something more than "VOR" as an approach title, so they get a letter.  Each circling (only) approach gets a different letter, so you could have, say, a VOR-A approaching from the north, a VOR-B from the south, and then a VOR/DME-C from the north, etc.  The ILS-X/Y/Z Rwy ## or RNAV-X/Y/Z Rwy ## differentiate different straight-in approaches, while the VOR-A/B/C etc are for circling-only approaches.